This application is based on Japanese Patent Application No. 09-064360 filed on Mar. 18, 1997, the contents of which are incorporated herein by reference.
1. Field of the invention
This invention relates to an apparatus for reading an image, and has particular reference to an apparatus capable of reproducing the obverse side of paper such that a copy taken is free of the adverse effect of characters, etc. printed on the reverse side of the paper and seen therethrough.
2. Description of the prior art
Books and magazines commonly in print consist of leaves, each of which has characters and/or charts printed on both sides of each leaf. In a known apparatus of the kind indicated above, it has been found that, when a book or magazine consists of thin leaves, the apparatus reads not only characters, etc. printed on the obverse side of a leaf but also characters, etc. printed on the reverse side of the leaf and seen therethrough. This will inevitably cause an impairment of picture quality.
Two methods of image information processing have been previously proposed to reduce the effects of characters, etc. printed on the reverse side of thin paper. One of these two methods is described in Japanese Laid Open Patent Application No. 7-87295, which discloses a copying machine adapted to copy both sides of an original. In this prior art method, a specific picture element contained in an image of the obverse side of the original is regarded as representing a portion of an image of characters, etc. printed on the reverse side of the original if (1) the photographic density of the specific picture element is less than a prescribed value and (2) if the photographic density of a picture element contained in an image of the reverse side of the original and corresponding to the specific picture element is greater than another prescribed value. The other of the two methods involves preparing a histogram for the distribution of the luminances of picture elements. For the purpose of correction, the luminances of picture elements read from the upper surface are calculated from the shape of the histogram.
A drawback of the first of the two methods is that this method can be implemented only in a copying machine adapted to copy both sides of an original. On the case of a copying machine in which an original has to be laid prone on an original glass plate and a copy of only one side of the original can be taken at one time, it is difficult to ensure the correct mating of the position of the original after turning-up with that before turning-up. Especially when one double-page spread after another of a book or magazine is an object to be read, difficulty is encountered all the more because, every time a leaf is to be turned over, the book or magazine has to be turned-up and positional again to lie prone on the original glass plate. In the case of a book scanner wherein a book or magazine is mounted on a baseboard so as to lie face up, difficulty is likewise encountered because, every time a leaf has been turned over, the positional relationship between the characters, etc. and the baseboard changes. Furthermore, the first of the two methods requires two image memories for storing data read from both sides of paper respectively. Obviously, this requirement has imposed a serious economic disadvantage to the first of the two methods.
With the second of the two methods, it has been found that a lightly printed or written image or photograph such as characters, etc. written in pencil or in cinnabar is frequently confused with that darkly printed or written on the reverse side of thin paper and is erased by mistake.